This invention relates to nondestructive testing and more particularly to that type of nondestructive testing in which it is desired to examine a test object or material to determine its structural integrity, such as for example, to determine the existence of cracks, voids, fissures, separations, foreign materials and inclusions as well as the dimensions of such imperfections. In such test systems, an energy source such as an x-ray generator is located at a predetermined distance from the object under test and the energy is directed through the test object to an imaging device, for example, a film sensitive to x-rays, to form an image of the object under test; in this example, a radiological image. It is desirable in such testing to have a means (called an image quality indicator (IQI)) to assess the quality of the (radiological) image formed. Radiological image quality comprises contrast sensitivity and spacial resolution. Contrast sensitivity is a measure of the minimum percentage change in an object which produces a perceptible density (brightness) change in the radiological image. Spacial resolution is a measure of the minimum size of an area of differing density (brightness) within an object which can be perceived on the radiological image.
In the past, radiological image quality was determined through the use of plack and wire penetrameters. Plack penetrameters measure contrast sensitivity and are fabricated as a flat sheet in different material or sheet thicknesses and contain a number of holes of various sizes through the thickness of the sheet. Wire penetrameters are fabricated by mounting lengths of material of various diameters on a flat sheet of material and are intended to measure spatial resolution. When used as radiological image quality indicators, the plack penetrameter and the wire penetrameter are deployed on the object to be tested and energy from a source (x-ray radiation) is directed through the penetrameters and the test object to an imaging device (x-ray film). Interpretation of the image formed on the imaging device provides an indication of the image quality. Additional information on radiological image quality indicating may be found in the 1997 Annual Book of ASTM Standards un Volume 03.03 Nondestructive Testing.
One problem with the prior art image quality indicators is that when the test object is relatively small, it is difficult to mount both a plack penetrameter and a wire penetrameter on the test object to obtain both contrast sensitivity and spatial resolution simultaneously. Further, since a wire penetrameter places additional material in the energy (x-ray) path, the quality is degraded, particularly when testing for voids, cracks, fissures and the like where it is the absence of material that is being tested for.
Accordingly, it is an object of this invention to provide an image quality indicator which determines contrast sensitivity and spatial resolution simultaneously.
It is a further object of this invention to provide an image quality indicator which provides a more accurate determination of image quality.